Recursive serialization of attributes.

In a D6 site, I found a couple of entries in the content_field_link table where field_link_attributes is a string over 40,000 characters long, all recursive serializations of an initial two character string.
... "s:88:"s:80:"s:72:"s:64:"s:56:"s:48:"s:40:"s:32:"s:24:"s:16:"s:9:"s:2:"N;";";";";";";";";"; ...

Checking if this value is a string in _link_process() before serialization prevents this.

A patch is attached for D7 which would also be good for D6.